Automotive lighting units and lamp assemblies are commonly used on vehicles today. Lighting units are typically used on the interior of the vehicle for general area illumination or as localized light sources, commonly referred to as map lamps or reading lamps. Automobile headlamps are used to illuminate the roadway for night driving and for enhancing vehicle visibility during daytime driving, especially when rainy or foggy conditions are present. Additionally, brake lamps and signal lamps are used to communicate a driver's intended actions to drivers of other road vehicles as well as pedestrians.
As shown in FIGS. 1–3, a conventional lighting device 1 includes a lamp or bulb 2, a reflector 3 and a lens 4. As shown in FIG. 2, the reflector 3 reflects the light from the lamp 2 so that the light rays 5 are reflected toward the lens 4. The direction of the reflected light may be altered depending on the shape and overall design of the reflector 3 and lens 4.
Primarily, these lighting units and lamp assemblies rely on incandescent (vacuum or gas filled) lamps as illumination sources. These lamps are often used in conjunction with a metallic reflector in order to increase the output of the lighting devices by reflecting a significant portion of the light produced by the lamp filament toward the desired illumination target zone, which is unique for each lighting application. In many cases, increased demands for safety and customer satisfaction have resulted in a need for greater light output for automotive lighting units and lamp assemblies. In response to customer demands for greater light output from these devices, lighting unit designers have typically sought to incorporate more powerful light sources (typically incandescent vacuum or gas filled lamps) and more efficient lighting reflectors and lens designs.
Typically, the reflectors are constructed from a thin stamped aluminum substrate or from a thin reflective coating, such as aluminum deposited onto a substrate such as glass or a polymer, such as acrylic or polycarbonate, which has been pre-formed into the desired reflector shape. When using a traditional metallic reflector, such as aluminum, a significant portion (typically about 90 percent) of light in the visible region, with a wavelength range of approximately 400 to approximately 700 nanometers, can be reflected. Thus, with the appropriate design of the reflector shape, about 90 percent of the visible light emitted by the lamp filament that strikes the reflector can be directed through the lens of the lighting unit toward the target illumination zone. Through advanced engineering techniques, such as computerized ray-tracing algorithms, efficient reflector shapes can be derived which maximize the amount of light that can be directed through the lens, out of the lighting unit, and toward the target zone. In addition to increased efficiencies gained through optimized reflector designs, more powerful lamps are also used to generate more light at the target illumination zone in or around the vehicle.
Although the above-identified efforts to increase light output from automotive lamps and lighting units have been effective in their primary design objective, they have brought about an unwanted effect, namely, increased heating of the lamp or lighting unit lens member. Such an increase in lens temperature causes several unwanted and/or problematic results. First, lighting unit lenses which are used for map and reading lamp functions are exposed to the interior portion of the vehicle, whereby the outer surface of the lighting unit lens may be touched by vehicle occupants. In order to protect against pain or injury caused by touching an excessively hot surface, automobile manufacturers have specified that the lens temperature must remain below a predetermined safe temperature threshold. The maximum allowable lens temperature varies by automobile manufacturer, but is often in the range of approximately 40 to approximately 65 degrees Celsius.
An additional problem that can occur due to the excessive heating of the lens of an automotive lighting unit is deformation, discoloration, and/or burning of the polymeric lens substrate. This can mean that the light output of a lighting unit may indeed be limited by the mechanical properties of the lens material. Thus, for automotive headlamp applications, increasing the light output of the lamp assembly may limit the choice of suitable lens materials to a glass substrate, which may add more weight as well as cost to a vehicle over a light assembly having a polymeric lens. The thermal stability of polymeric lighting unit lens members is also an issue for map and reading lamp assemblies as well as for brake and signal lamps. Designers of such lighting units must ensure through analysis and testing that the lens member will not undergo any visible deformation, discoloration, and/or burning—even after being continuously used for hundreds of hours. For this reason, the light output of the lamp assembly may be limited by the temperature stability limits of the lens member.
One reason that directing an increased amount of light in the visible region of the spectrum through the lens member of a lighting unit leads to increased temperature in the lens can be explained by considering the full spectrum of radiation emitted by the lamp filament. Using traditional metallic reflectors, any attempt to direct greater amounts of visible light (wavelengths of approximately 400 to 700 nanometers) through the lens will inevitably also direct a larger percentage of radiation in the infrared wavelengths (greater than approximately 700 nanometers) through the lens. When this radiation contacts the inner surface of the lens of the lighting unit, a large percentage (shown generally by the lines 6 in FIGS. 2 and 3) of the incident radiation (typically on the order of 90 percent) will be transmitted through the lens. A small amount (shown generally by the line 7 in FIG. 3) of the incident radiation (about 4 to 5 percent) will be reflected off the interior lens surface back into the lighting unit, and another approximately 5 to 10 percent (shown generally by the line 8 in FIG. 3) of the incident radiation (averaged across all wavelengths emitted by the lamp filament) of the radiation will be absorbed by the lens. The absorption of the radiation by the lens member will cause an increase in temperature of the lens itself. Thus, the more visible light which is directed through the lens, the more the lens will be heated by the associated infrared energy which is absorbed by the lens.
This is best understood by considering that the light source of the lamp filament acts as a black body emitter, whose spectral radiation output varies as a function of source temperature. The temperature of the filament source for incandescent (vacuum and gas filled) lamps typically range between approximately 2,400 and approximately 3,000 degrees Kelvin. Considering, for example, the idealized black-body radiation curve for a lamp filament at 3,000 degrees Kelvin (FIG. 4), it can be seen that a small band of emissive power occurs in the visible wavelengths but a much broader band of radiation occurs in the infrared region. Since the reflectivity of traditional metallic reflectors are approximately constant across the visible and infrared regions, it is clear that any attempt to increase visible light output of the lighting unit either by more efficient reflector design geometry or by more powerful light sources, will also increase the amount of infrared radiation that is absorbed by the lens, resulting in increased lens temperature. For this reason, the light output of such a device will likely be limited by the problematic, but as of yet unavoidable, increase in lens temperature.